Emulsion polymerization of vinyl acetate



Patented Dec. 15, 1953 UNITED STATES ATENT OFFICE EMULSIONPOLYMERIZATION OF VINYL ACETATE tion of Delaware No Drawing. ApplicationDecember 14, 1950, Serial No. 200,871

6 Claims.

This invention relates to the emulsion polymerization of vinyl acetateand more particularly to the preparation of an improved aqueousdispersion of polyvinyl acetate.

Stable aqueous dispersions of polyvinyl acetate ordinarily calledpolyvinyl acetate emulsions are commonly made by dispersing monomericvinyl acetate in water in the presence of a suitable dispersing agent,generally with the aid of rapid stirring or other suitable agitation andpolymerizing the dispersed monomer by the addition of a polymerizationcatalyst at a suitable temperature or by subjecting it to otherpolymerizing conditions, for example, by exposure to actinic radiation.The art is replete with a variety of methods for carrying out such emulsion polymerizations to obtain the so-called emul- 'ons which arerelatively stable aqueous dispersions of solid polyvinyl acetateexisting in the form of very fine particles.

Such vinyl acetate emulsions are useful for a variety of purposes,including adhesive and coat ing uses. The emulsions, when spread upon asolid surface and permitted to dry, form contin uous films which areoften transparent. Such films spread between two solid bodies andpermitted to dry there form an excellent adhesive bond. For such coatingand adhesive uses, the emulsions may be mixed with other ingredients,solid or liquid, to serve as fillers, pigments, reinforcing agents,extenders, plasticizers and solvents. For example, by suitable additionof pigments and solvents, the emulsions may be utilized in water-basepaint formulations.

In the uses of the polyvinyl acetate emulsions as coatings andadhesives, difliculties have arisen because of the lack of adequatewater resistance. The presence of the dispersing agent utilized inmaking the emulsion, is mainly responsible for the lack of waterresistance in films obtained from the emulsion. The dispersing agenttends to cause the redispersion of the polymer thus imparting to thefilms the effect of water solubility to a greater or less extent.

Various methods have been proposed heretofore for reducing the Watersensitivity of films obtained from polyvinyl acetate emulsions. Onemethod is to treat the film during or after its formation with asubstance which chemically reacts with the dispersing agent to destroyits effectiveness as a dispersing agent. In many emulsions which utilizewater soluble grades of polyvinyl alcohol as dispersing agents, watersensitivity can be reduced more or less by treating the emulsion duringor after a coating process with a material such as an aldehyde or thelike which will insolubilize the polyvinyl alcohol, thereby destroyingits dispersing activity. Such methods have the disadvantage of requiringadditional operational steps such as treating the coated articles ormixing in a treating agent just prior to application of the vinyl resinemulsion. Another method comprises adding to the vinyl resin emulsionmaterials capable of insolubilizing the polyvinyl alcohol or otherdispersing agent to the emulsion. This method has been only partiallysuccessful for the reason that generally such reagents tend to reactwith the dispersing agent while the emulsion is in storage or shipment,thus causing coagulation. It has long een desired to produce a stablepolyvinyl acetate emulsion which can be stored and shipped forindefinite periods of time without coagulation and which, when spreadinto a film and dried at temperatures of 20 to 25 C. or higher, form afilm which does not redisperse when contacted with Water.

For many uses of the polyvinyl acetate emulsions high dilution stabilityis a very desirable property. In many applications, for example, inapplying the vinyl resin to textile and paper products it is desired todilute the emulsion with water to as low as 1 to 3% polymerconcentration. Generally, it is not possible to dilute polyvinyl acetateemulsions to' a polymer content of less than about as at lowerconcentrations the emulsion becomes unstable and the vinyl resinparticles tend to coagulate or settle out to an excessive extent.Consequently for applications requiring high dilutions, it has beennecessary to add a protective colloid or thickening agent or tocontinuously agitate the diluted emulsion, or both.

An important characteristic of a polyvinyl acetate emulsion is thesolids content, that is the proportion of the polyvinyl resin to thewater in the emulsion. A high solids content generally is to bepreferred as such a product, particularly if it has high dilutionstability, is useful for many purposes requiring varying solids contentover a wide range. It is generally difiicult to increase the solidscontent of a vinyl resin dispersion without sacrificing other desirableproperties such as high stability, resistance of films to the action ofwater and the like. Heretofore it has been difficult if not impossibleto prepare emulsions having more than around 30 to 40% of polyvinylacetate which have a high degree of stability during storage andshipment and which form films having a, high degree of resistance towater. It has been particularly difiicult to pro- 3 duce suchdispersions with solids content above 40% by weight of polyvinylacetate, which have a high degree of mechanical stability.

By the term mechanical stability as used herein, we mean the stabilityof a dispersion against coagulation when subjected to the action or"mechanical force such as stirring, shaking or other forms of mechanicalagitation. Often dispersions have a high degree of stability and willnot coagulate on standing for indefinite periods of time so long as thedispersion stands quiescent and the dispersed particlesare subjectedt'ono exterior mechanical forces, except the force of gravity. Many suchdispersions, however, when agitated as by stirring, flowing through pipelines or subjected to the agitation normally occurring intransportation, tend to coagulate under the influences of suchmechanical forces. For most commercial uses, high mechanical stabilityis an important and desired property for polyvinyl acetate emulsions.

An object of this invention is an improved process for the production ofpolyvinyl acetate emulsion. A further object is to prepare apolyvinyl'acetate emulsion, stableduring storage and shipment, from,which substantially completely water-resistant films can be castWithout the necessity of chemical or heat treatments. Another object isto prepare polyvinyl acetate emulsions having the 'above'properties andalso-a high solids content. Another object is to prepare polyvinylacetate emulsions having 'high dilution stability and high mechanicalstability. Still other objects of the invention will be apparent fromthe f allowing description.

The above objects may be attained in accordance with the presentinvention'by the herein described method for =-polymerizing' vinyl Iacetate. This 'method'depends'upon the use of a completelyhydrolyzed'grade of polyvinyl alcohol (or certain other polyhydrox-ypolymeric film-forming' substance hereinafter described) as thedispersing agent, the utilization of a certain technique in carrying outthe 'polymerization and the presence, duringat least "apart-ofthe-polymerization reaction, of a"cyclic alcohol selected from the group"consisting of thebterolsfabietyl alcohol, dehydroab'ietylalcoholandhydroabietyl alcohols.

In one methodfor-polymerizingvinyl acetate in accordance with ourinvention, '"we may first dissolve a completely hydrolyzed grade of"polyvinyl alcohol in hot waterto form asolut ion containing from '45 to6% by Wightbf thep'olyvinyl alcohol. Wethen'add asmallamount of vinylacetate not exceeding 1.5 to 3%"of the weight of the aqueous solutionanddi'sperse this by adequate agitation. "Polymerizationcatalyst is thenadded and the dispersion, withbon'tinue'd agitation, is' brought to asuitable temperature, for example, '82 to 90 C. so as'to'cause'polymerization of the dispersed vinyl acetate. At any timeafter the polymerization 'i'eaction'has started, including a timesufficient to completely polymerize the initial quantity of vinylacetate, further small quantities :of vinyl acetate are added, eithercontinuously or intermittently, provided that the amount of vinylacetate added or the rate of addition of vinyl acetate is 'such that theamount of vinyl acetate in the reaction mixture never exceeds 5% of theweight o'f-the aqueous portion, so long as the amount of polymer formeddoes not exceed %01 the weight of said aqueous portion. When the amountof 'of polymer formed has reached 30 to 55% by Weight of the mixture. Asthe polymerization progresses, further amounts of catalyst may be added,as required to maintain a suitable cata-' lyst concentration.

The pH of the reaction mixture must be com trolled and at the start ofthe reaction mixture must be above 4, e. g., at a pH of 6 to 8. Duringthe reaction the pl-I will fall, due to some hyd'r'olysis of vinylacetate to form acetic acid. At coinrile'tion of the polymerization thepH will be less than 6, e. g., around 3 to 4.

Before or during the polymerization reaction we add to the reactionmixture a small amount of cyclic alcohol, for example, cholesterol, inamount equal to not less than about 0.1% by weight of the water presentin the completed dispersion. The cyclic alcohol may be added eitherbefore or during the polymerization reaction, provided that the alcoholis added before the polymerization is more than 50% complete. By percentcomplete, we mean the proportion of vinyl acetate which has beenpolymerized based on the total that is to be polymerized. For example,if the completed polyvinyl acetate emulsion is to contain 50% by weightof polyvinyl acetate, the polymerization is considered 50% complete whenhalf of the required amount of vinyl acetate has beenpolymerized, thatis, when the polymer concentration reaches about 33%. Preferably, atleast part of the cyclic alcohol is added at the start of thepolymerization. A convenient method is to dissolve the required amountof the alcohol in the vinyl acetate to be polymerized. Or, if desired,all or a part of the alcohol-may be added to the water before theaddition'of any vinyl acetate.

The amount of the cyclic alcohol added may vary over a wide range,depending on the individual alcohol and the properties desired. For bestresults generally the proportion of the cyclic alcohol should be fromabout 0.25% of the Weightof the water in the finished emulsion up to 5times the limit of solubility of the alcohol in the water present. Stilllarger amounts can be used to produce emulsions having good mechanicalstability, but the presence of such large amounts of undissolved alcoholtends to lower the quality of the product in regard to properties suchas screening tests, smoothness and clarity of films cast from theemulsion, and the like. In making emulsions'having a polyvinyl acetatecontent within the range of 40 to60%, the amount of cyclic alcoholemployed is preferably from about 0.04 to about 0.4% of the total weightof the finished emulsion.

The resulting polyvinyl acetate dispersions are latex-like or creamycompositions containing up to '55 to 60% by weight of polyvinyl acetate,which have high mechanical stability and which yield clear, transparent,substantially water insoluble films 'of vinyl acetate when spread onsolid' surfaces and dried at temperatures of 20 to 25 C. or higher.

We have found that such non-redispersible film-forming emulsions cannotbe made (1) by using a partially hydrolyzed grade of polyvinyl alcoholas the emulsifying agent, regardless of the rate of monomer addition nor(2) by using Characterization procedures Particle siee.The particle sizeof the emulsion is determined by examination of a highly diluted sampleunder a microscope equipped with a calibrated scale.

pH.-Beekmann pH meter (measured at C.).

' Viscosity.-The viscosity is determined at 25 C. with a Brookfieldviscosimeter, with the spindle rotating at 60 R. P. M. and reported incentipoises (cp.).

Air-dried film-A 0.008" thick wet film is cast on a 7 x 17" ground glassplate with a Boston Bradley adjustable blade doctor knife. It is placedin a constant temperature room at 72 F. and 72% relative humidity for 24hours. This film is used for observation of clarity, redispersion andnumber of oscillations-wet rubs.

Redispersion.Redispersion is noted when the above film is wetwith waterand rubbed with a nylon brush; it the water becomes milky, redispersionis stated to occur.

Number of oscillations-wet rubs.-The above indicated air-dried film istested according to the Federal Specifications TTP-88a (with themodification that water instead of soap solution is used and no heattreatment of film is permitted) with a Gardner Model 105 washability andabrasion machine. In this test the film is scrubbed with a wet-weightednylon brush. The number of oscillations of this wet scrubbing is notedwhen any area of the central 4 section of the film fails. A test of 4000oscillations or better indicates excellent wet abrasion resistance. Thedesignation OK indicates no observable failure of the film.

Per cent screen test.A 100-gram sample of the emulsion is diluted withWater to 1000 grams and is run through a weighed 200 mesh per sq. in.screen. The screen is washed until the filtrate is clear. It is thendried in the oven at 100-105 C. to constant weight. (An emulsion of 0.1%or less screen test on 200 mesh screen is acceptable to most users ofpolyvinyl acetate emulsions. In many applications, an emulsion with ascreen test of 0.2% or greater would have to be screened before use.)

6% solids dilation settling test.-The polyvinyl acetate emulsion isdiluted with water to 6% solids and 100 cc. of the diluted emulsion isallowed to stand for 24 hours. A reading is taken of the number of cc.of sludge settled out. (This is an indication of dilution stability. Anemulsion of less than 2 cc. can be used by most emulsion consumers whoapply the emulsion in a dilute form. An emulsion which contains morethan 3' cc. of sludge in 100 cc. of 6% solids would be unsatisfactoryfor dilute application.)

Mechanical stability test.Place 200 grams of sample in a 500 ml. beakerand with a stainless steel agitator running at 300-600 R. P. M. agitatesample at 20-30 C. until coagulation occurs or for 24 hours if nocoagulation occurs. The beaker is cylindrical, about 2 inches indiameter by 6 inches high. The agitator is a two-bladed propeller 1 inchlong 1( o.5' inch radius) by 0.5 inch wide, mounted on a vertical shaftabout 0.25 inch in diameter. The mechanical stability is unsatisfactoryif coagulation occurs in less than 8 hours. Mechanical stability is fairto good if coagulation occurs within the 8: to 16 hour interval andexcellent if no coagulation occurs after 20 hours agitation.

EXAMPLE 1 Materials Water g 640 Completely hydrolyzed grade of polyvinylalcohol g 40 NaHCO; g 1.2 Vinyl acetate"; g 460 Zinc formaldehydesulfoxylate (8%) cc 25.4 H202 (4%) cc 25.2

Polymerization The polyvinyl alcohol and sodiumvbicarbonate were mixedwith the water in a vessel fitted with an agitator and reflux condenser.This mixture was then stirred and heated at to for about one hour. Vinylacetate was slowly added until a mild reflux was obtained (about 1.3%vinyl acetate) to displace oxygen which inhibits polymerization.Hydrogen peroxide (4%) and zinc formaldehyde sulfoxylate (8%) wereadded, in equal amounts, at a rate which would give a. satisfactorypolymerization. The vinyl acetate was added at a rate which wouldmaintain a steady refiux with the reactor at 85 to 87 C. and maintainmonomer content within the range of 0.5 to 2% by weight. The polyvinylacetate emulsion was then cooled to about 25 C. with mild stirring. Thepolymerization time was about two hours.

The procedure of Example 1 wasfollowed, except that the polymerizationtemperature was 84 to 87 C. and the time was 1.75 hours.

Characterization Polyvinyl acetate content 55%. I Pgrticle size l ldainly ca. 1-2 microns. p .o. Viscosity 1 320 cp. Air-dried film(floudy, rough. Redispersion one. Screen test Over 1%.

6% solids dilution settling test; 1.5 cc. Number of oscillations-wetrubs OK at 5.000.

Mechanical stability test Coagulates in less than 1 minutes.

2,062,863 7 EXAMPLE 8' Characterization M t i l gofllrygcilneygiggetatecontent 55%. water g 7 Range of spheres Prreriliongnantly 0.2-0.3

or n. Completely hydrolyzed grade Of polyvinyl 5 \TllXiIll;1m spheres(except ir- Up to 1.5 microns. gfggg "i" H 1.5555555 158.410! 6 microns.Vinyl acetateIIIIIIIIIIIIIQI es x gggg' g g g e Cholesterol g Screentest 0.0055%. z formaldehyde sulfoxylate 19 Mechanical stability test OKat 8 hours. H20: (4%) cc 21 EXAMPLE 6 Polymerization Materials Water g700 The cholesterol was dissolved in the vinyl acetate before adding thelatter to the reaction mix- 23 2 23 hydrolyzed grade 34 ture. Thereaction temperature was 8485 C. NaHCOa 15 and the time ofpolymerization was 3 hours. -'7"; Otherwise, the procedure was the sameas in gg i zg ggfi alcohol Abltol "2%" Example Zinc formaldehydesulfoxylate (6%) cc 39 Charactenzation H202 (4%) cc 39 Polyvinyl acetatecontent 40%. Polymerization Particle size:

Predominant spheres Range of spheres 0.1 micron. 1-3 microns.

pH 4.4. Viscosity 980 cp. Air-dried film l Clear. Redlspersion None.

Screen test 0.012%. Mechanical stability test- OK at 8 hours.

EXAMPLE 4 Materials Water g 700 Completely hydrolyzed grade of polyvinylalcohol g 24 NaHCO: g 1.5 Cholesterol g 1.0 Vinyl acetate g 920 Zincformaldehyde sulfoxylate (8%) cc 40 H20: (4%) cc 40 Polymerization Thecholesterol was added to the. aqueous portion separately andsimultaneously with the first addition of vinyl acetate. The reactiontemperature was 85-88? C. and the reaction time was 2.5 hours.Otherwise, the procedure was the same as in Example 1..

Characterization Polyvinyl acetate content. 55%. Particle size:

Range of spheres Predominantly 0.2-0.5

micron. Maximum spheres So mo ca. 3.0 microns.

Th pr dure of Example 3 was. empl yed. x cept. that the reactiontemperature, was 82-85 C. and' thereaction time was 3.5 hours.

The procedure of Example 3 was employed, dissolving the Abitol in thevinyl acetate. (Abitol is a hydrogenated abietyl alcohol productsupplied by Hercules Powder Company, 82% by weight of which consists ofparts by weight of tetrahydroabietyl alcohol and 15 parts ofdihydroabietyl alcohol.) The reaction time was 2.5

hours and the reaction temperature was 83-85" C.

Characterization Polyvinyl acetate content 55%. Particle size:

Mainly ca. 0.2 micron.

Some to 3.0 microns.

Some ro(1s" ca. 3 x 15 micron. pH .3. Viscosity 0.400 cp Air-dried filmClear. Screen test 0.011%. 6% solids dilution test Trace. Mechanicalstability test OK at 12 hours.

EXAMPLE 7 Materials Water cc 700 Completely hydrolyzed grade ofpolyvinyl alcohol g 24 NaI-ICO: g 1.5 Cholesterol g 1.0 Hydroabietylalcohol (Abitol) g 4.0 Vinyl acetate cc 1,000 Zinc formaldehydesulfoxylate (6%) cc 44 H20: (4%) cc 44 Polymerization The procedure ofExample 6 was employed. Reaction temperature was 84-86 0.; reactiontime, 3.5 hours.

sion made with a completely hydrolyzed grade of polyvinyl alcohol.

Example 3. is substantially the same as Example 1, except for theaddition of cholesterol. Thisaddition reduces the predominant particlesize from 015 micron to 0.1 micron and the screen test from 0.27% to0.012% in the 40% solids emulsion.

7 Example 2 is similar to Example 1, except that the solids contents israised to 55% and the ratio of polyvinyl alcohol to water is less tokeep the viscosity down to a workable range. The resulting emulsion ismechanically unstable, the average particle size more than doubles andthe screen test is very high. However, Example 5 shows that theemployment of cholesterol in making this 55% solids emulsion results insmall particle size, good mechanical stability and ex- "celler'i'tscreen test.

The cholesterol can most eiiectively be used if :present in thepolyvinyl alcohol solution at the beginning of vinyl acetate additions,as shown by Example l, where the amount employed is only' about onesixth of that in Example'B.

The polyvinyl alcohols suitable as emulsifiers in practicing ourinvention are those which are made by reacting polyvinyl acetate with analcohol or water until at least 93% of the acetate groups have beenconverted to hydroxyl groups. We prefer to use polyvinyl alcohols whichare thus 99 to 100% saponified- Such polyvinyl alcohols are herein andin the appended claims termed completely hydrolyzed grade of polyvinylalcohol.

The above described completely hydrolyzed grade of polyvinyl alcohol asgenerally made is insoluble or ony slightly soluble in cold water, i.e., at temperatures below 50 0.; and in order to prepare an aqueoussolution thereof for the practice of this invention we usually stir amixture of water and the polyvinyl alcohol at a temperature above 50. C.until a substantially clear solution is found. Preferably, we stir themixture to disperse the polyvinyl alcohol (in finely divided form) inthe water and heat the dispersion at 80 to 95 C. for about one hour, toobtain a clear solution. Such solution may be cooled, to roomtemperature, without precipitation of dissolved polyvinyl alcohol.

Certain polyhydroxy polymeric film-forming substances other than thecompletely hydrolyzed grades of polyvinyl alcohol may also be used asdispersing agents in our process, for example, certain starches andhydroxy alkyl celluloses. To serve as a substitute for'completelyhydrolyzed grades of polyvinyl alcohol in this process, the polymericmaterial must contain hydroxyl groups in proportions equal to or greaterthan that of other functional groups (such as ether groups, estergroups, carboxyl, sulfonyl and the like) and must also havesubstantially the water solubility characteristics described above. Thatis, it must be substantially insoluble Or only slightly soluble in waterat temperatures below 50 C. but soluble when heated with water attemperatures at or above 50 C. (e. g., '70 to 100 (1.), to formsolutions. One example is any of the various known soluble starches,which must be heated to 70 or higher in contact with water to formaqueous solutions. Such soluble starch, suitable for our purpose, may bemade, for example, by'partially sulfonating starch, so that less thanhalf of the hydroxyl group of the starch are sulfonated.

Other forms of modified starch containing a preponderance of freehydroxyl groups and having the above solubility characteristics may beused as well.

The hydroxy alkyl cellulose (hydroxy alkyl cellulose ethers) which havethe above described solubility characteristics, e. g., hydroxy ethylcellulose, also may be usedin place of the completely hydrolyzed gradesof polyvinyl alcohol to practice our invention.

The concentration of dispersing agent in the polymerization reactionmixture will vary, depending on its dispersing power and the viscosityof its aqueous solution. For example, a completely hydrolyzed grade ofpolyvinyl alcohol, a 4% by weight aqueous solution of which has aviscosity less than centipoises at 20 C., will be employed as a 4.5 to6% by Weight aqueous solution. Still higher concentrations, e. g., 6 to10%, may be employed if desired. Other completely hydrolyzed grades ofpolyvinyl alcohol whose aqueous solutions have higher viscosities may beused in lower concentrations, e. g., around 2%. In general theconcentrations of dispersing agent in the practice of our invention doesnot depart from conventional practice in emulsion polymerization ofvinyl acetate, sufiicient dispersing agent being employed to form astable dispersion of the resulting polymer.

The catalyst preferably used in our process is the type we termactivated peroxide type polymerization catalyst, by which term we meanthe combination of a peroxygen compound and a reducing agent, forexample, a bisulfite, sulfurous acid, sulfoxylate or other sulfoxycompound having reducing properties. The peroxygen com.- pound may behydrogen peroxide, an organic peroxide such as benzoyl peroxide, acetylperoxide or the like, an organic or inorganic peracid or a salt thereof,e. g., peracetic acid, perborates and persulfates; The best resultsgenerally are obtained by using a water soluble peroxygen compound, e.g., hydrogen peroxide or an inorganic persulfate or perborate. Also wegenerally prefer to add the peroxygen compound and the reducing compoundseparately to the polymerization reaction mixture. A preferred catalystis the combination of hydrogen peroxide with a sulfoxylate, e. g., zincformaldehyde sulfoxylate separately added. Such activated peroxygen typepolymerization catalysts are well known in the art and are described,for example, in Brubaker et al., U. S. P. 2,462,354. While we prefer toutilize the above activated peroxide type of catalyst, the invention isnot restricted thereto, as other catalyst suitable for the polymerizaionof vinyl acetate may be used, for, example, any of the various peroxygencompounds such as those mentioned above, with or without the addition ofreducing agents or other activating materials and the various azocompounds which have been more recently discovered to catalyze vinylacetate polymerization.

Theamount of catalyst and the temperature of the reaction mixture duringthe polymerization are not critical and may follow conventionalprocedures forthe polymerization of vinyl acetate. We generally preferto add the catalyst in small amounts simultaneously with the addition ofthe monomer, but such is not essential, so long as the catalystconcentration is maintained suificiently high to cause polymerization.

In practicing our invention to produce stable polyvinyl acetateemulsions which on drying at 20 to 25 C. form films highly resistant toredispersion in water it is essential that:

(l) The dispersing agent be either a completely hydrolyzed grade ofpolyvinyl alcohol as defined above or other polyhydroxy polymericsubstance in which free hydroxyl groups, are present in amount equal toor greater than other functional groups and which is soluble in water ata temperature above,:50;C. but :insoluble in water at temperaturesbelowi50 C.

(2) The amount of monomeric'vinyl acetate present in the reactionmixture does not exceed of the weight of the water present-when thepolymer content of the emulsion is by weight or less, and does notexceed 3% of the weight of the water when the polymer content is above20% by weight.

(3) The initial pH of the polymerization reaction mixture must be above4 and must be permitted to fall to a pH not-higher than ,6 beforecompletion of the reaction.

(4) The polymerization is continued-until the polymer content hasreached :at least 20% by weight but not more than about 55 to 60%.

'If any one of theabove four requirements is not met in thepolymerization procedure, the r..- sulting product will not be -a stableemulsion which forms water-resistant films on drying. For example, ifthe dispersing agent is a partially hydrolyzed grade of polyvinylalcohol, the resulting emulsion will not form water-resistant films,regardless of the regulation of -monomer content during polymerization.If the monomer content of the .reaction mixture initially exceeds 5% orexceeds 3% when the polymer content is above 20% during-thepolymerization reaction, the water resistance of films cast from theresulting product-will be unsatisfactory.

Proceeding in accordance with the above stated four essentialrequirements, dispersions which produce films not redispersible in watercan be made without the addition of the cyclic alcohol. However, withoutthe addition of a cyclic alcohol, e. g., cholesterol, before thepolymerization is more than 50% complete, the emulsion will not havehigh dilution stability or high mechanical stability. The dilutionstability can be evaluated by means of the 6% solids dilution settlingtest described above. As shown by the above examples, the addition ofthe cyclic alcohol markedly reduces the amount of solids settled out in24 hours from an emulsion diluted to 6% polymer concentration. Ingeneral the effect of the cholesterol or other suitable cyclic alcoholis to decrease the particle size of the dispersed polymer and also todecrease the amount of agglomerates. This ef feet is demonstrated by theabove described Per cent screen test. The presence of the 03- clicalcohol during polymerization also imparts to the resulting emulsion ahigh degree of mechanical stability as shown by the Mechanical stabilitytest.

Further, the hereindescribed 'use of the cyclic alcohol is essential toproduce emulsions having the above described properties, whilecontaining more than to 40% by weight of polyvinyl acetate, e. g., up toaround 55%.

In place of cholesterol we may use any other water-soluble sterol ormixtures of such sterols. Suitable sterols may be either of animalorigin or of plant origin (phytosterols). Examples of suitable sterolsare cholesterol, isocholesterol, oxychloesterol, agnosterol, lanosterol,sitosterol, dihydrositosterol, stigmasterol and ergosterol.

It is preferable to use the sterols in as pure state as possible toavoid contamination of the polyvinyl acetate emulsion with excessiveamounts of impurities such as fats and the like which may accompany thesterols in their commercial forms. However, the objects of thisinvention may be attained by using commercial essential feature of ourprocess.

12 sterol products such .ascommercial-soy sterols, commercial woolvfat-sterols, ox bile extract and the like.

In addition to the sterols, cyclic alcohols suitable for practicing ourinvention include abietyl alcohol, the hydroabietyl alcohols, thesulfonated hydroabietyl alcohols, and dehydroabietyl alcohol. We preferto use the hydroabietyl alcohols or commercial products containing them.A commercial product which is suitable for practicing the invention isAbitol a crude mixture of hydroabietyl alcohols produced by the HerculesPowder Company and which contains dihydroabietyl alcohol andtetrahydroabietyl alco- Control of .monomer content of thepolymerization reaction mixture is an important and During thereaction,the monomer content, regulated by the rate ,ofaddition of monomer, mustnot exceed 5% by weight of the reaction mixture when the polymer contentis 20% by weight or less and must not exceed.3% by weight when thepolymer content exceeds 20%. Preferably, we begin the reaction witharound 1 to 1.5% of vinyl acetate in the emulsion and maintain the vinylacetate content at.1.5 to 3% during most of the reaction. By employing anitrogen atmosphere to exclude air as described below, we maymaintainthe vinyl acetate monomer content at not over about 0.4%, whichgives the best product at reasonably low catalyst consumption. Ifdesired, the monomer content may be maintained as low as 0.1%. Tocontrol the monomer concentration, the vinyl acetate is addedsubstantially at the rate of .its polymerization and catalyst is addedcontinuously or intermittently as required to maintain reaction.

It is desirable to exclude air from the reaction mixture, as molecularoxygen has an inhibitory effect on the polymerization. Preferably, theair is excluded by operating in a closed reactor equipped with areflux-condenser and maintaining an atmosphere of vinyl acetate vaporover the reaction mixture. Alternatively, the reactor free space -may befilled with an oxygen-free, inert gas such as nitrogen.

Inpracticing our invention, for the best results the polyvinyl alcoholshould be substantially completely dissolved in the water before thevinyl acetate and polymerization catalyst are added. As most of thecompletely hydrolyzed grades of polyvinyl alcohol are difficult todissolve in cold water, we generally heat and stir a mixture of thepolyvinyl alcohol and water at 70 to C. until substantially completesolution is obtained.

It is essential that the pH of the polymerization reaction mixtureinitially be above 4 and we prefer an initial pH of around 6 to 8. Toinsure a reasonably high pH we generally prefer to add a mild alkalinebuffer such as sodium bicarbonate, disodium phosphate, sodium acetate,or the like. If the mixture initially contains acetic acid, it may beneutralized by adding the above alkali or a stronger alkali such assodium hydroxide, sodium carbonate or the like. As the polymerizationproceeds, the pH of the reaction mixture generally falls, due toformation of acetic acid, caused by hydrolysis of part of the vinylacetate monomer. Preferably, the initial pH is so adjusted (e. g., to pHof 6 to 8) so that the final pH is above 4. However, a good product maybe obtained at a final pH below 4, provided that during the greater partof the reaction the 1 3 DH is above 4. When a mild alkaline bufier suchas sodium bicarbonate or disodium phosphate is used to adjust theinitial pH, we prefer to add the bufier to the water before dissolvingthe polyvinyl alcohol therein.

If the initial pH is below 4, the resulting emulsion produces waterdispersible films. Low initial pH also tends to caus undue thickening ofthe emulsion, increase in particle size of the dispersed polymer,aggregation of particles and coagulation.

The polymerization temperature is not particularly critical andtemperatures conventionally employed for polymerizing vinyl acetate withperoxide catalyst are suitable, e. g., from 50 to 90 C. We generallyprefer to maintain the reaction mixture at reflux temperature, e. g., at85 to 90 C. In operating at lower temperatures without refluxing, weprefer to maintain a nitrogen atmosphere in the reactor to exclude air.

The emulsions prepared as described herein are aqueous dispersions ofsolid polyvinyl acetate having improved characteristics. Films made bydrying these emulsions at 20 to 25 C. or higher are substantiallyinsoluble (i. e., not redispersible) in water at any temperature. Theseemulsions are further characterized by unusually small particle size ofthe polyvinyl acetate, and relative freedom from agglomerates. They havea high degree of stability both in concentrated and diluted forms and inparticular, have a high degree of mechanical stability. The emulsionsmay be made with solids (polyvinyl acetate) content as high as 55 to60%, substantially without diminution or impairment of the above statedproperties. The emulsions are useful for a variety of purposes,particularly for coating solid sur faces, and as adhesives. For suchpurposes the emulsions may be used per se or in combination with otheringredients such as pigments, extenders, fillers, plasticizers,colorants and the like.

We claim:

1. The process which comprises dispersing vinyl acetate monomer in anaqueous 2 to 10% by weight solution of a hydrophilic dispersing agentwhich is a polyhydroxy, polymeric, filmforming substance substantiallyinsoluble in water at temperatures below 50 C. but soluble in water at atemperature not lower than 50 C. to form a dispersion containing notmore than by weight of said monomer, subjecting said dispersion topolymerization in the presence of a peroxygen type catalyst, whiledispersing therein further quantities of said monomer so as to maintaina monomer concentration not greater than 5% by weight when the polymerconcentration is not over 20% by weight and not greater than 3% byweight when the polymer content has exceeded 20% by weight, continuingas above until the polymer content has reached 20 to 60% by weight,controlling the pH of the polymerization reaction mixture so that it isinitially not less than 4 and finally not greater than 6 and adding tothe reaction mixture before polymerization is 50% complete a cyclicalcohol selected from the group consisting of the sterols, abietylalcohol, dehydroabietyl alcohol the hydroabietyl alcohols and thesulfonated hydroabietyl alcohols in amount equal to not less than about0.1% by weight of the water present.

2. The process which comprises dispersing vinyl acetate monomer in anaqueous 2 to 10% by weight solution of a completely hydrolyzed grade ofpolyvinyl alcohol to form a dispersion containing not more than 5 byweight of said monomer, subjecting said dispersion to polymerization inthe presence of a peroxygen type catalyst, while dispersing thereinfurther quantities of said monomer so as to maintain a monomerconcentration not greater than 5% by weight when the polymerconcentration is not over 20% by weight and not greater than 3% byweight when the polymer content has exceeded 20% by weight, continuingas above until the polymer content has reached 20 to 60% by weight,controlling the pH of the polymerization reaction mixture so that it isinitially not less than 4 and finally not greater than 6 and adding tothe reaction mixture before polymerization is 50% complete a cyclicalcohol selected from the group consisting of the sterols, abietylalcohol, dehydroabietyl alcohol, the hydroabietyl alcohols and thesufonated hydroabietyl alcohols in amount equal to not less than about0.1% by weight of the water present.

3. The process which comprises dispersing vinyl acetate monomer in anaqueous 2 to 10% by weight solution of a completely hydrolyzed grade ofpolyvinyl alcohol in concentration of said.

monomer equal to 0.1 to 3% by weight, subjecting the resultingdispersion to polymerization in the presence of peroxygen typepolymerization catalyst and adding and dispersing therein furtheramounts of said monomer in such quantities that the monomerconcentration never exceeds a certain value, depending upon theconcentration of dispersed polymer, as follows: not over 5% when thepolymer concentration is not over 20% by weight; not over 3% when thepolymer concentration is more than 20% by weight; continuing saidaddition of monomer until the resulting dispersion contains 20 to about55% by weight of polymer, controlling the pH of the polymerizationreaction mixture so that it is initially not less than 4 and finally notmore than 6 and adding to the reaction mixture before polymerization is50% complete a cyclic alcohol selected from the group consisting of thesterols, abietyl alcohol, dehydroabietyl alcohol, the hydroabietylalcohols and the sulfonated hydroabietyl alcohols in amount equal from0.25% by weight of the water present up to 5 times the limit ofsolubility of the alcohol in the water.

4. The process according to claim 3 in which the polymerization catalystis an activated peroxide type and the cyclic alcohol is cholesterol.

5. The process according to claim 3 in which the polymerization catalystis an activated peroxide type and the cyclic alcohol is abietyl alcohol.

6. The process according to claim 3 in which the polymerization catalystis an activated peroxide type and the cyclic alcohol is hydroabietylalcohol.

JOHN E. BRISTOL. WARREN A. HARRINGTON. NORRIS TURNBULL.

References Cited in the file of this patent UNITED STATES PATENTS OTHERREFERENCES Warth, The Chemistry and Technology of Waxes, Reinhold 1947,pp. and 81.

1. THE PROCESS WHICH COMPRISES DISPERSING VINYL ACETATE MONOMER IN ANAQUEOUS 2 TO 10% BY WEIGHT SOLUTION OF A HYDROPHILIC DISPERSING AGENTWHICH IS A POLYHYDROXY, POLYMERIC, FILMFORMING SUBSTANCE SUBSTANTIALLYINSOLUBLE IN WATER AT TEMPERATURES BELOW 50* C., BUT SOLUBLE IN WATER ATA TEMPERATURE NOT LOWER THAN 50* C. TO FORM A DISPERSION CONTAINING NOTMORE THAN 5% BY WEIGHT OF SAID MONOMER, SUBJECTING SAID DISPERSION TOPOLYMERIZATION IN THE PRESENCE OF A PEROXYGEN TYPE CATALYST, WHILEDISPERSING THEREIN FURTHER QUANTITIES OF SAID MONOMER SO AS TO MAINTAINA MONOMER CONCENTRATION NOT GREATER THAN 5% BY WEIGHT WHEN THE POLYMERCONCENTRATION IS NOT OVER 20% BY WEIGHT NOT GREATER THAN 3% BY WEIGHTWHEN THE POLYMER CONTENT HAS EXCEEDED 20% BY WEIGHT, CONTINUING AS ABOVEUNTIL THE POLYMER CONTENT HAS REACHED 20 TO 60% BY WEIGHT, CONTROLLINGTHE PH OF THE POLYMERIZATION REACTION MIXTURE SO THAT IT IS INITIALLYNOT LESS THAN 4 AND FINALLY NOT GREATER THAN 6 AND ADDING TO THEREACTION MIXTURE BEFORE POLYMERZATION IS 50% COMPLETE A CYCLIC ALCOHOLSELECTED FROM THE GROUP CONSISTING OF THE STEROLS, ABIETYL ALCOHOL,DEHYDROABIETYL ALCOHOL THE HYDROABIETYL ALCOHOLS AND THE SULFONATEDHYDROABIETYL ALCOHOLS IN AMOUNT EQUAL TO NOT LESS THAN ABOUT 0.1% BYWEIGHT OF THE WATER PRESENT.